Formatting a storage device, like a hard drive or block storage, creates a filesystem on the device (and erases any existing data). You typically only need to format a device once, when you first create it.

Partition volumes to divide a single storage device into smaller units that you can manage independently.
After you resize a partitioned volume to increase its available amount of disk, rewrite the partition table and expand the filesystem to use the new space.
Create a new volume from the control panel, format it to create its filesystem, then attach and mount it to a Droplet to use it.